My 2002 XJR has been exhibiting a CEL for a while, along with a rather intermittent “Restricted Performance” (RP) message. C0172 and C0175 were indicating both banks as “too rich”. After reading all I could to educate myself along with reading other fellows’ experiences, I thought something common to both banks must be the problem. Although the MAFS had been replaced only about a year ago as part of that nice $4,000.00 repair bill by this local British shop in SoCal, I still suspected the MAFS, so I removed it, inspected and cleaned it in spite of the fact that it was only a year old. Put it back in the car and P0172 and P0175 were gone and have not returned.
Then I had to resolve two more codes, P1638 and P1642. The main thing that I have learned about these codes is that they come up when there is a communication problem between the ECM and the instrument cluster. After carefully studying the circuit schematics I realized that we’re talking about only two wires that, on their way to the instrument cluster, they do go through a lighting module inside the tranny selector tower, but for now I just left that alone. I rather proceeded to just clean the ECM output connector pins (BTW, no humidity or water or signs of ever being wet in there) and the instrument cluster’s black connector pins, although I did clean the pins on the yellow connector, as well, just because. Put everything back and guess what? Yup, codes P1638 and P1642 were now gone as well, and still are. Oh, and the CEL is gone, too, of course (finally).
Uhh, the “Restricted Performance message is gone, as well, but I went too fast with this and I failed to identify which one of the two sets of codes is the one that got rid of the “RP” announcement. Probably it was the open lead(s).
I thought of adding this info here because this resolved these issues for me cheap and fast, meaning you don’t need to examine fuel trims or any expensive parts before you clean up those 20 years old connections.